MB-100: _wait_warmup_completed after upgrade

Change-Id: Ibc461ef92cdce2ad14740e6463279eab5b35385f
Reviewed-on: http://review.couchbase.org/25556
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>